BOSTON (Reuters) - Opponents of same-sex marriage face near-certain defeat in their bid to overturn a ruling in Massachusetts allowing gays and lesbians to wed, a result that could influence debate on the hotly contested issue across the country.

Just over a year after Massachusetts became the first U.S. state to allow gay couples to marry, political support is fading for a state constitutional amendment that would ban gay marriage and allow only same-sex civil unions.

Massachusetts lawmakers vote on the proposal on September 14 in a constitutional convention. Approval would pave the way for a final hurdle -- a state referendum on the amendment in 2006. But a senior lawmaker expressed doubts it would get that far.

House Speaker Salvatore F. DiMasi said rejection was likely.

"At this point he thinks the way it looks now, the amendment will not proceed any further," said DiMasi spokeswoman Kim Haberlin.

DiMasi is the latest Massachusetts politician to signal dying political and public support for overturning the ground-breaking ruling by the state's highest court that struck down a ban on gay marriage as unconstitutional.

Advocates say more than 6,000 same-sex couples have been issued marriage licenses in Massachusetts.

But they see more fights ahead that could determine whether other states follow Massachusetts's lead, as conservatives and Christian groups keep pushing for an outright ban on both same-sex marriages and civil unions in the state.

Many gay rights activists oppose civil unions, which lack some of the federal benefits of a legal marriage.

IMMINENT CHALLENGE

The biggest challenge could come from a separate initiative calling for a state referendum in 2008 to ban gay marriage and not allow civil unions. The state's attorney general must decide by September 7 whether to certify or reject that.

"We believe we are on rock-solid ground," said Kristian Mineau, president of the conservative Massachusetts Family Institute that is leading the initiative.

If it gets a green light, Mineau's group would need to gather 66,000 signatures in a petition supporting the ban. Then they must win backing from at least 50 lawmakers in two legislative sessions to put it on the ballot.

That means more standoffs with activists such as Lee Swislow, executive director of Gay and Lesbian Advocates and Defenders, which is circulating its own petition of lawyers that says the challenge is legally flawed.

"We think the Massachusetts Constitution says pretty plainly that you cannot use a ballot initiative process to reverse a judicial decision. So we think we have a pretty strong argument," said Swislow.

Among her more powerful supporters is the head of the Boston Bar Association, Ellen Carpenter, who joined about 80 other lawyers in signing the petition before sending it the attorney general with nearly 50 pages of supporting documents.

"As far as I understand it, the provision cannot be amended to overturn a supreme judicial decision," Carpenter said. "And that letter is signed by real heavy hitters in the Boston and Massachusetts legal community."

But for some gay couples, like 37-year-old Amy Wyeth and her long-term partner Valerie who plan to marry in November, the frenzied debate is not enough to sway their wedding plans.

"It's been amazing to me that I have not encountered any prejudice or problems," said Wyeth, although she expressed some concern over the risk of the laws being reversed.

"It concerns me, but I also think that once psychologically you have something in place, it's harder to take that away."